The invention relates to a circuit for attenuating noise signals in a FM receiver, wherein the output signal of a FM demodulator is applied to an absolute value function signal generator from which is derived a control signal for an attenuator which increases the attenuation in the signal channel above a threshold value of the absolute value of the demodulator output signal.
Such a circuit is described in the German Offenlegungsschrift No. 2,602,908.
In this German Offenlegungsschrift the amount generator consists of a rectifier bridge to the output of which a capacitor is connected. Due to the relatively high demodulator output resistance and a series resistor, the capacitor can, however, only be charged relatively slowly. Also discharging the capacitor can only be effected slowly, so that the capacitor, whose voltage switches an attenuator through an operational amplifier, is charged to the average value of the demodulator output voltage. This average value constitutes the so-called AFC-voltage, that is to say a voltage which is proportional to the deviation between the center frequency of the FM demodulator and the carrier frequency of a received transmitter. It is, however, not possible to eliminate all the different kinds of disturbing noise signals with this circuit. The disturbing noise signals can be divided into three groups:
(a) disturbing noise signals which are produced when no transmitter signal is received, that is to say when, within the passband of the intermediate frequency amplifier of the receiver, no transmitter signal is received or only transmitter signals which are so weak that the useful low-frequency signal obtained is disturbed too much by the noise.
In such a case the signal channel supplies a strong noise being only little lower than the signal obtained in the case of correct tuning to a correct received transmitter signal which is modulated with a maximum deviation. This type of disturbing noise will be denoted as noise surging hereinafter.
The known circuit cannot suppress this surge of noise by means of the circuit described in the opening paragraph. Therefore, a voltage which is derived from the received field strength is here additionally used, the attenuation in the signal channel being increased when the field strength decreases.
(b) Disturbing noise signals which are produced when a transmitter signal is received on the edges of the intermediate frequency filter. This is shown in FIG. 1. FIG. 1 is a schematical representation of the curve 1 of the intermediate frequency amplifier response. The central frequency is at the frequency f0. When a transmitter having a carrier frequency f1 or f2 is received, considerable distortions are produced which may be heard considerably louder than the signal at a correct tuning to this transmitter carrier frequency.
This type of disturbing noise signals, which will be denoted as spurious reception hereinafter, can be effectively suppressed by means of the known circuit, as a d.c. voltage is generated, being dependent of the deviation between f0, f2 and f0, f1, respectively, which can be used to switch off the signal channel.
(c) Disturbing noise signals which are produced when the carrier frequencies of two transmitter signals coincide with the filter edges of the intermediate frequency filter (in this case (FIG. 1) the carrier frequency of one transmitter is at, for example f1 and the carrier frequency of the other transmitter at, for example, f2). This type of disturbing noise signals, which will be denoted intermediate reception hereinafter, can only be partly suppressed by the known circuit as, on the one hand, the received field strength is large whereas, on the other hand, the AFC-signal which is generated on tuning to a frequency located in f1 and f2--depending on the position of f1 and f2 and of the received field strength relative to f0--can always take the value zero (that is to say the AFC-voltage passes through zero and changes its sign during tuning), so that at least in this case the intermediate reception becomes audible when tuning the receiver.